This proposal is designed to merge the candidate's background in molecular biology and clinical experience in hematology/oncology to produce an independent physician-scientist investigator. Dr. A. Thomas Look, the candidate's mentor, is a recognized leader in leukemia research and has committed major resources at the Dana-Farber Cancer Institute to use the zebrafish as a model to study human myeloid cell development. In addition, an advisory committee of highly regarded medical scientists will provide scientific and career advice. The goal of this application is to use the genetic and embryonic advantages of the zebrafish to determine the pathophysiologic significance in hematopoiesis and leukemia of constitutive activating mutations in the class III receptor tyrosine kinase FLT3 gene, which is mutated in 20-30% of acute myeloid leukemia (AML) patients. The central hypothesis is that AMLs are caused by the synergistic action of specific classes of mutations acting in tandem. These include, first, an activated tyrosine kinase gene for stimulation of growth/survival (Class I) and, second, either in activation of a gene required for normal myeloid differentiation or activation of a chimeric transcription factor oncogene that can produce a block of myeloid cell development (Class II). Dr. Hsu will define the spectrum of phenotypes, including a potential proliferative and survival advantage, induced by constitutive expression of the FLT3-ITD tyrosine kinase mutant in developing zebrafish myeloid cells (Aim #l). He will also establish stable transgenic lines of zebrafish that over-express FLT3-ITD in myeloid stem and progenitor cells and monitor these fish for the development of myeloproliferative syndrome and AML (Aim #2). The development of stable transgenic lines is critical for testing the central hypothesis. These stable lines are also vital for the investigator's future goal of conducting modifier screens to discover additional important genes in the hematopoietic pathways used by the activated FLT3 kinase. Insight into this pathway may lead to discovery of shared components of the pathway in the 60 to 70 percent of AMLs that do not have a FLT3 mutation and the identification of genes that can serve as potential drug targets.